When administered to humans, mouse antibodies are generally recognized as foreign matters, inducing production of anti-mouse immunoglobulin antibodies in the human body. It is known that the former antibodies react with the latter antibodies to produce adverse effects [J. Clin. Oncol., 2, 881 (1984); Blood, 65, 1349 (1985); J. Natl. Cancer Inst., 80, 932 (1988); Proc. Natl. Acad. Sci. U.S.A., 82, 1242 (1985)] and that the mouse antibodies undergo rapid clearance [J. Nucl. Med., 26, 1011 (1985); Blood, 65, 1349 (1985); J. Natl. Cancer Inst., 80, 937 (1988)], thus showing only a reduced efficacy [J. Immunol., 135, 1530 (1985); Cancer Res., 46, 6489 (1986)]. Attempts have been made to solve these problems by deriving, from mouse monoclonal antibodies, chimeric human antibodies or CDR (complementarity determining region)-transplanted antibodies (reshaped antibodies) using gene engineering technique. In a human chimeric antibody, the variable regions thereof are of mouse origin and the constant regions thereof are of human origin [Proc. Natl. Acad. Sci. U.S.A., 81, 6851 (1984)] and it is reported that when administered to humans, said antibody causes litte human anti-mouse immunoglobulin antibody production, its blood half-life being 6-fold longer [Proc. Natl. Acad. Sci. U.S.A., 86, 4220 (1989)]. The CDR-grafted antibodies are antibodies resulting from replacement of the CDRs in a human antibody alone with the CDRs from an animal other than the human [Nature, 321, 522 (1986)] and, in an experiment with monkeys, such antibodies showed reduced immunogenicity and 4- to 5-fold higher serum half-lives as compared with mouse antibodies [J. Immunol., 147, 1352 (1991)].
As regards the cytocidal activity of antibodies, it is reported that the Fc region of a human antibody is more potent in activating human complement and human effector cells than the Fc region of a mouse antibody. Thus, for instance, a chimeric antibody derived from a mouse monoclonal antibody to the ganglioside GD.sub.2 and containing a human antibody Fc region enhances the human effector cell-mediated antitumor effect [J. Immunol., 144, 1382 (1990)]. Similar results are reported for CDR-grafted antibodies [Nature, 332, 323 (1988)]. Such results indicate that, for clinical use, humanized monoclonal antibodies are preferred to mouse monoclonal antibodies.
The antibody classes include IgA, IgM, IgG, IgD and IgE and , in mice, the class IgG includes four subclasses, namely IgG.sub.1, IgG.sub.2 a, IgG.sub.2 b and IgG.sub.3 (in humans, IgG.sub.1, IgG.sub.2, IgG.sub.3 and IgG.sub.4). When antigens are administered to animals, the antibodies produced mostly belong to the classes IgM or IgG. IgG molecules have a molecular weight of about 160,000 daltons and a dimeric structure and are relatively easy to handle. IgM molecules are large with a molecular weight of about 900,000 daltons and occur in the form of a complicated pentameric structure coupled with the joining (J) chain, hence they have the following drawbacks: they are difficult to purify; they tend to agglutinate, hence are difficult to store; they are readily inactivated by partial decomposition in the presence of a protease, hence it is difficult to prepare Fab fragments; and they lose their binding activity in many instances upon chemical modification, for example chemical binding of an anticancer agent or a toxin [J. W. Goding: Monoclonal Antibodies: Principles and Practice, Academic Press, 1986]. As to which are superior in therapeutic effect against cancer, IgG class monoclonal antibodies or IgM class monoclonal antibodies, reference may be made to a detailed study made by Bernstein et al. using an IgG class monoclonal antibody and an IgM class monoclonal antibody to the lymphocyte Thy-1 antigen [Monoclonal Antibodies, edited by R. H. Kennet, T. J. Mckearn and K. B. Bechtol, Plenum Press, 1980, p. 275]. According to the reference, an IgG class monoclonal antibody and an IgM class monoclonal antibody comparable in terms of reactivity to Thy-1 antigen-positive lymphocytes, were compared in terms of antitumor effect. While the IgM monoclonal antibody was superior in in vitro complement-dependent antitumor effect, the IgG class monoclonal antibody showed a significant antitumor effect in in vivo antitumor effect in cancer-bearing mice, with no antitumor effect being observed with the IgM class monoclonal antibody. It was further revealed that, as compared with the IgG class monoclonal antibody, the IgM class monoclonal antibody showed a very short half-life in the blood after administration, in an isotope-labeled form, to mice. These results indicate that the monoclonal antibodies to be used clinically in humans should preferably be of the IgG class.
Gangliosides, a class of glycolipids, are constituents of animal cell membranes. These molecules are composed of a carbohydrate chain, which constitutes a hydrophilic side chain, and sphingosine and a fatty acid, which constitute hydrophobic side chains. It is known that the ganglioside species expressed and the amount thereof differ between cell species, organ species, and animal species, among others. Furthermore, it has been reported that the ganglioside expressed changed quantitatively and qualitatively during the process of cancer development [Cancer Res., 45, 2405 (1985)]. For example, expression of the gangliosides GD.sub.2, GD.sub.3 and GM.sub.2 has been reported in neuroblastoma, lung small cell carcinoma, and melanoma, which are highly malignant neural ectodermal tumors [J. Exp. Med., 155, 1133 (1982); J. Biol. Chem., 257, 12752 (1982); Cancer Res., 47, 225 (1987); ibid., 47, 1098 (1987); ibid., 45, 2642 (1985); Proc. Natl. Acad. Sci. U.S.A., 80, 5392 (1983)].
GM.sub.2, one of the gangliosides that are sialic acid residue containing glycolipids, occurs only in trace amounts in normal cells but is found in increased amounts in cancer cells in lung small cell carcinoma, melanoma, neuroblastoma, etc. Monoclonal antibodies to GM.sub.2 are considered to be useful in the treatment of such cancers [Lancet, 4, 786 (1989)]. However, those monoclonal antibodies to GM.sub.2 that have so far been reported are of the human IgM class or of the rat IgM, mouse IgM or mouse IgG class [Cancer Res., 46, 4116 (1986); Proc. Natl. Acad. Sci. U.S.A., 79, 7629 (1982); Cancer Res., 48, 6154 (1988); J. Biol. Chem., 264, 12122 (1989)].
Anti-ganglioside GM.sub.2 monoclonal antibodies, if produced in the form of humanized antibodies, for example chimeric human antibodies or CDR-grafted antibodies, which are not expected to induce anti-mouse immunoglobulin antibody production in the patient's body, produce reduced adverse effects and show a prolonged blood half-life and an enhanced antitumor effector effect. These antibodies are thus expected to be superior in therapeutic effect to the corresponding mouse monoclonal antibodies.